Contactless uniform-tunneling separate P-well (CUSP) non-volatile memory array architecture, fabrication and operation

ABSTRACT

Floating-gate field-effect transistors or memory cells formed in isolated wells are useful in the fabrication of non-volatile memory arrays and devices. A column of such floating-gate memory cells are associated with a well containing the source/drain regions for each memory cell in the column. These wells are isolated from source/drain regions of other columns of the array. Fowler-Nordheim tunneling can be used to program and erase such floating-gate memory cells either on an individual basis or on a bulk or block basis.

RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.10/655,251, filed Sep. 4, 2003 now U.S. Pat. No. 6,930,350 and titled,“CONTACTLESS UNIFORM TUNNELING SEPARATE P-WELL (CUSP) NON-VOLATILEMEMORY ARRAY ARCHITECTURE, FABRICATION AND OPERATION,” which is adivisional of U.S. patent application Ser. No. 10/230,597, filed Aug.29, 2002 now U.S. Pat. No. 6,649,453 and titled, “CONTACTLESS UNIFORMTUNNELING SEPARATE P-WELL (CUSP) NON-VOLATILE MEMORY ARRAY ARCHITECTURE,FABRICATION AND OPERATION,” both of which are commonly assigned andincorporated by reference in their entirety herein.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to non-volatile memory cells andin particular the present invention relates to flash memory cells.

BACKGROUND OF THE INVENTION

Memory devices are available in a variety of styles and sizes. Somememory devices are volatile in nature and cannot retain data without anactive power supply. A typical volatile memory is a DRAM which includesmemory cells formed as capacitors. A charge, or lack of charge, on thecapacitors indicate a binary state of data stored in the memory cell.Dynamic memory devices require more effort to retain data thannon-volatile memories, but are typically faster to read and write.

Non-volatile memory devices are also available in differentconfigurations. For example, floating gate memory devices arenon-volatile memories that use floating gate transistors to store data.The data is written to the memory cells by changing a threshold voltageof the transistor and is retained when the power is removed. Thetransistors can be erased to restore the threshold voltage of thetransistor. The memory may be arranged in erase blocks where all of thememory cells in an erase block are erased at one time. Such non-volatilememory devices are commonly referred to as flash memories.

The non-volatile memory cells are fabricated as floating gate memorycells and include a source region and a drain region that is laterallyspaced apart from the source region to form an intermediate channelregion. The source and drain regions are formed in a common horizontalplane of a silicon substrate. A floating gate, typically made of dopedpolysilicon, is disposed over the channel region and is electricallyisolated from the other cell elements by a dielectric. For example, agate oxide can be formed between the floating gate and the channelregion. A control gate is located over the floating gate and is can alsobe made of doped polysilicon. The control gate is electrically separatedfrom the floating gate by another dielectric layer. Thus, the floatinggate is “floating” in dielectric so that it is insulated from both thechannel and the control gate.

As semiconductor devices get smaller in size, designers are faced withproblems associated with the production of memory cells that consume asmall enough amount of surface area to meet design criteria, yetmaintain sufficient performance in spite of this smaller size.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art foralternative memory device architectures.

SUMMARY

The above-mentioned problems with non-volatile memory cells and otherproblems are addressed by the present invention and will be understoodby reading and studying the following specification.

The various embodiments relate to non-volatile semiconductor memorycells, arrays, as well as their fabrication and architecture. Suchmemory cells can use Fowler-Nordheim (FN) tunneling during both programand erase operations while maintaining random access capabilities. Dueto the nature of FN tunneling, the memory cells can operate atrelatively low power consumption. Additionally, because of the low powerconsumption of FN tunneling compared to hot-electron processes, many,e.g., thousands, of cells may be programmed or erased in parallel. Whileparallel programming and erase operations are suitable for large blocksof memory cells, cells may be programmed or erased individually whilestill facilitating a smaller cell size than a typicalelectrically-erasable programmable read-only memory (EEPROM).

For one embodiment, the invention provides an array of floating-gatefield-effect transistors. The array includes two or more columns of thefloating-gate field-effect transistors, each field-effect transistor ofa column sharing a first source/drain region and a second source/drainregion with other field-effect transistors of that column. The first andsecond source/drain regions of a column are contained in a first wellhaving a first conductivity type. The first well for each column isisolated from first wells of other columns.

For another embodiment, the invention provides a method of erasing amemory cell in an array of memory cells. The method includes applying afirst potential to a word line associated with the memory cell, applyinga second potential to a first source/drain region and a secondsource/drain region of the memory cell, and applying the secondpotential to a first well containing the first and second source/drainregions. The method further includes applying a third potential to asecond well. The second well is underlying the first well and is coupledto the first well through a PN junction.

For yet another embodiment, the invention provides a method ofprogramming a memory cell in an array of memory cells. The methodincludes applying a first potential to a word line associated with thememory cell, applying a second potential to a first source/drain regionand a second source/drain region of the memory cell, and applying athird potential to a first well containing the first and secondsource/drain regions. The method further includes applying the thirdpotential to a second well underlying the first well. The second well iscoupled to the first well through a PN junction and the third potentialhas the second polarity.

For still another embodiment, the invention provides a non-volatilememory device. The memory device includes an array of non-volatilefloating-gate memory cells arranged in rows and columns and controlcircuitry for controlling access to the array of memory cells. Eachcolumn of memory cells shares a source and a drain, the source and drainfor a column of memory cells being contained in a first well associatedwith that column of memory cells. The first well associated with eachcolumn of memory cells is isolated from other first wells of othercolumns of memory cells. Each first well is overlying a second well in amany-to-one relationship and each first well has a first conductivitytype. The second well has a second conductivity type different than thefirst conductivity type.

Further embodiments of the invention include methods and apparatus ofvarying scope.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic of an array of memory cells formed in accordancewith one embodiment of the invention.

FIG. 1B is a block diagram of a non-volatile memory device in accordancewith one embodiment of the invention.

FIG. 2A is a planar view of an array of field-effect transistors (FETs)in accordance with one embodiment of the invention.

FIG. 2B is a cross-sectional view of a portion of the array of FETs ofFIG. 2A.

FIGS. 3A–3F are cross-sectional views of a portion of a memory arrayduring various stages of fabrication in accordance with one embodimentof the invention.

DETAILED DESCRIPTION

In the following detailed description of the invention, reference ismade to the accompanying drawings that form a part hereof, and in whichis shown, by way of illustration, specific embodiments in which theinvention may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilizedand structural, logical, and electrical changes may be made withoutdeparting from the scope of the present invention.

To aid in the interpretation of the detailed description and claims thatfollow, the term “semiconductor substrate” is defined to mean anyconstruction comprising semiconductive material, including, but notlimited to, bulk semiconductive materials such as a semiconductor wafer(either alone or in assemblies comprising other materials thereon) andsemiconductive material layers (either alone or in assemblies comprisingother materials). The term “substrate” refers to any supportingstructure, including, but not limited to, the semiconductor substratesdescribed above. The term substrate is also used to refer tosemiconductor structures during processing, and may include other layersthat have been fabricated thereupon. Both wafer and substrate includedoped and undoped semiconductors, epitaxial semiconductor layerssupported by a base semiconductor or insulator, as well as othersemiconductor structures well known to one skilled in the art.

In addition, as the structures formed by embodiments in accordance withthe present invention are described herein, common semiconductorterminology such as n-type, p-type, n+ and p+ will be employed todescribe the type of conductivity doping used for the various structuresor regions being described. The specific levels of doping are notbelieved to be germane to embodiments of the present invention; thus, itwill be understood that while specific dopant species and concentrationsmay not be mentioned, an appropriate dopant species with an appropriateconcentration to its purpose, is employed.

The term conductor is understood to also include semiconductors, and theterm insulator is defined to include any material that is lesselectrically conductive than the materials referred to as conductors.The following detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is defined onlyby the appended claims, along with the full scope of equivalents towhich such claims are entitled.

Finally, it will be understood that the number, relative size andspacing of the structures depicted in the accompanying figures areexemplary only, and thus were selected for ease of explanation andunderstanding. Therefore such representations are not indicative of theactual number or relative size and spacing of an operative embodiment inaccordance with the present invention.

FIG. 1A is a schematic of an array 100 of memory cells 101 in accordancewith an embodiment of the invention. The memory cells 101 are supportedby a substrate 102. Each memory cell 101 is an FET including a controlgate provided in FIG. 1A by a word line 120, a floating gate 116, afirst source/drain region 108 and a second source/drain region 110. Onesource/drain region acts as a source while the remaining source/drainregion acts as a drain for the FET.

A column of memory cells 101 is defined as those memory cells 101sharing the same source/drain regions 108 and 110, e.g., memory cells101 ₁₁ and 101 ₁₂ are in the same column. A row of memory cells 101 isdefined as those memory cells 101 sharing the same word line 120, e.g.,memory cells 101 ₁₁ and 101 ₂₁ are in the same row. Although only twocolumns and two rows are depicted in FIG. 1A, a typical array maycontain hundreds and even thousands of rows and columns.

The body of each memory cell 101 in a column of memory cells 101 iscoupled to a first node 106. A first node 106 of a first column ofmemory cells 101 is isolated from other first nodes 106 of other columnsof memory cells 101 within the array 100, in part by a second node 112having a different conductivity type than the first nodes 106. The firstnodes 106 are each coupled to the second node 112 through a PN junction.

Table 1 demonstrates operations to the memory array 100 for oneembodiment of the invention. Although specific potentials are listed inTable 1, it will be appreciated by those skilled in the art that otherpotentials can be specified to create the necessary voltage differentialto exceed a threshold voltage of a target memory cell during a readoperation or to provide the necessary voltage differentials tofacilitate FN tunneling to add charge to, or remove charge from, thefloating gate 116 of a target memory cell. As such, the variousembodiments are not limited to the specific potentials listed in Table1.

TABLE 1 Bias Conditions during Array Operations (Assuming memory cell101₁₁ as the Target Memory Cell) Op Read Erase Program Erase ProgramNode Target Target Target All All 120₁ Vwr V1 V2 V1 V2 108₁ Vdr V2 V1 V2V1 110₁ 0 V V2 V1 V2 V1 106₁ 0 V V2 V1 V2 V1 120₂ 0 V 0 V 0 V V1 V2 108₂0 V 0 V 0 V V2 V1 110₂ 0 V 0 V 0 V V2 V1 106₂ 0 V 0 V 0 V V2 V1 112 0 V0 V V1 0 V V1 102 0 V 0 V 0 V 0 V 0 VAppropriate potentials are generally in the range of approximately 5V to15V for V1 and approximately −5V to −15V for V2.For Table 1, node 120 ₂ represents all word lines 120 for rows notcontaining the target memory cell 101 ₁₁, node 108 ₂ represents allfirst source/drain regions 108 for columns not containing the targetmemory cell 101 ₁₁, node 110 ₂ represents all second source/drainregions 110 for columns not containing the target memory cell 101 ₁₁,and node 106 ₂ represents all first nodes 106 for columns not containingthe target memory cell 101 ₁₁.

As shown in Table 1, for one embodiment, reading the target memory cell101 ₁₁ of the array 100 may be performed by applying a read voltage Vwr,e.g., 4.5V, to the word line 120 ₁ of the target memory cell, applying abias Vdr, e.g., 1V, to a first source/drain region 108 ₁ of the targetmemory cell and detecting a current at the first source/drain region 108₁, or current or potential at the second source/drain region 110 ₁. Theread voltage is some voltage in excess of the threshold voltage for theconductive cell that will not cause a read disturb, i.e., a change of adata value of the cell. During this read operation, the first node 106 ₁of the target memory cell and the second node 112 are each brought to aground potential. Remaining word lines 120 ₂, first source/drain regions108 ₂, second source/drain regions 110 ₂, first nodes 106 ₂ and thesubstrate 102 are also brought to a ground potential.

Erasing typically refers to bringing a group of memory cells to auniform state, i.e., a first logic state, usually in preparation for anensuing programming operation. As shown in Table 1, for one embodiment,erasing the target memory cell 101 ₁₁ of the array 100 may be performedby applying a first programming voltage having a first polarity (V1),e.g., 8V, to the word line 120 ₁ of the target memory cell and applyinga second programming voltage having a second polarity (V2), e.g., −8V,to a first source/drain region 108 ₁, second source/drain region 110 ₁and first node 106 ₁ of the target memory cell. Remaining nodes, i.e.,the second node 112, word lines 120 ₂, first source/drain regions 108 ₂,second source/drain regions 110 ₂, first nodes 106 ₂ and the substrate102 are brought to a ground potential. To erase all memory cells of thearray 100, the word lines 120 ₂, are brought to the first programmingvoltage instead of the ground potential and the first source/drainregions 108 ₂, second source/drain regions 110 ₂ and first nodes 106 ₂are brought to the second programming voltage instead of the groundpotential.

Programming typically refers to a final step in a process of bringing amemory cell or group of memory cells to physical states, i.e., secondlogic states, that represent a data pattern to be stored in the array.Thus, a data pattern may be stored in the array 100 by placing all ofthe memory cells in the first logic state and then placing one or morememory cells in the second logic state. As shown in Table 1, for oneembodiment, programming the target memory cell 101 ₁₁ of the array 100may be performed by applying the second programming voltage having thesecond polarity (V2), e.g., −8V, to the word line 120 ₁ of the targetmemory cell and applying the first programming voltage having firstpolarity (V1), e.g., 8V, to a first source/drain region 108 ₁, secondsource/drain region 110 ₁ and first node 106 ₁ of the target memorycell, as well as the second node 112. Remaining nodes, i.e., word lines120 ₂, first source/drain regions 108 ₂, second source/drain regions 110₂, first nodes 106 ₂ and the substrate 102 are brought to a groundpotential. To program all memory cells of the array 100, the word lines120 ₂ are brought to the second programming voltage instead of theground potential and the first source/drain regions 108 ₂, secondsource/drain regions 110 ₂ and first nodes 106 ₂ are brought to thefirst programming voltage instead of the ground potential.

Table 2 demonstrates operations to the memory array 100 for anotherembodiment of the invention in which voltages of a single polarity areused. Although specific potentials are listed in Table 1, it will beappreciated by those skilled in the art that other potentials can bespecified to create the necessary voltage differential to exceed athreshold voltage of a target memory cell during a read operation or toprovide the necessary voltage differentials to facilitate FN tunnelingto add charge to, or remove charge from, the floating gate 116 of atarget memory cell. As such, the various embodiments are not limited tothe specific potentials listed in Table 2.

TABLE 2 Bias Conditions during Array Operations (Assuming memory cell101₁₁ as the Target Memory Cell) Op Read Erase Program Erase ProgramNode Target Target Target All All 120₁ Vwr Vhi Vlo Vhi Vlo 108₁ Vdr VloVhi Vlo Vhi 110₁ 0 V Vlo Vhi Vlo Vhi 106₁ 0 V Vlo Vhi Vlo Vhi 120₂ 0 VVmed or Vlo Vmed Vhi Vlo 108₂ 0 V Vmed Vmed or Vlo Vhi Vlo 110₂ 0 V VmedVmed or Vlo Vhi Vlo 106₂ 0 V Vmed Vmed or Vlo Vhi Vlo 112 0 V Vmed VhiVlo Vhi 102 0 V 0 V 0 V 0 V 0 VAppropriate potentials are generally in the range of approximately 0V to1V for Vlo, 6V to 12V for Vmed and 12V to 30V for Vhi.For Table 2, node 120 ₂ represents all word lines 120 for rows notcontaining the target memory cell 101 ₁₁, node 108 ₂ represents allfirst source/drain regions 108 for columns not containing the targetmemory cell 101 ₁₁, node 110 ₂ represents all second source/drainregions 110 for columns not containing the target memory cell 101 ₁₁,and node 106 ₂ represents all first nodes 106 for columns not containingthe target memory cell 101 ₁₁.

As shown in Table 2, for one embodiment, reading the target memory cell101 ₁₁ of the array 100 may be performed by applying a read voltage Vwr,e.g., 4.5V, to the word line 120 ₁ of the target memory cell, applying abias Vdr, e.g., 1V to a first source/drain region 108 ₁ of the targetmemory cell and detecting a current at the first source/drain region 108₁, or a current or potential at the second source/drain region 110 ₁.The read voltage is ome voltage in excess of the threshold voltage forthe conductive cell that will not cause a read disturb, i.e., a changeof a data value of the cell. During this read operation, the first node106 ₁ of the target memory cell and the second node 112 are each broughtto a ground potential. Remaining word lines 120 ₂, first source/drainregions 108 ₂, second source/drain regions 110 ₂, first nodes 106 ₂ andthe substrate 102 are also brought to a ground potential.

As shown in Table 2, for one embodiment, erasing the target memory cell101 ₁₁ of the array 100 may be performed by applying a first programmingvoltage (Vhi), e.g., 16V, to the word line 120 ₁ of the target memorycell and applying a second programming voltage (Vlo), e.g., 0V, to afirst source/drain region 108 ₁, second source/drain region 110 ₁ andfirst node 106 ₁ of the target memory cell. The second node 112, firstsource/drain regions 108 ₂, second source/drain regions 110 ₂ and thefirst nodes 106 ₂ are brought to a third programming voltage (Vmed),e.g., 8V, between the first and second programming voltages. The wordlines 120 ₂ are brought to either the second or third programmingvoltage. The substrate 102 is brought to a ground potential. To eraseall memory cells of the array 100, the word lines 120 ₂, are brought tothe first programming voltage instead of the second or third programmingvoltage, and the first source/drain regions 108 ₂, second source/drainregions 110 ₂, first nodes 106 ₂ and second node 112 are brought to thesecond programming voltage instead of the third programming voltage.

As shown in Table 2, for one embodiment, programming the target memorycell 101 ₁₁ of the array 100 may be performed by applying the secondprogramming voltage (Vlo) to the word line 120 ₁ of the target memorycell and applying the first programming voltage (Vhi) to a firstsource/drain region 108 ₂, second source/drain region 110 ₁ and firstnode 106 ₁ of the target memory cell, as well as the second node 112.Remaining word lines 120 ₂ are brought to the third programming voltage(Vmed), and the first source/drain regions 108 ₂, second source/drainregions 110 ₂ and first nodes 106 ₂ are brought to the secondprogramming voltage (Vlo) or the third programming voltage (Vmed). Thesubstrate 102 is brought to the ground potential. To program all memorycells of the array 100, the word lines 120 ₂ are brought to the secondprogramming voltage instead of the third programming voltage and thefirst source/drain regions 108 ₂, second source/drain regions 110 ₂ andfirst nodes 106 ₂ are brought to the first programming voltage insteadof the second or third programming voltage.

Although in the foregoing description, programming and erase operationswere described with reference to the operation of injecting electronsfrom the floating gate to the channel and the operation of injectingelectrons from the channel to the floating gate, respectively, theseoperations are exchangeable. Accordingly, memory cells with a high Vt ornon-conductive state can represent either cells in a programmed state oran erased state.

FIG. 1B is a block diagram of a non-volatile or flash memory device 160in accordance with one embodiment of the invention. The memory device160 is coupled to a processor 161 to form part of an electronic system.The memory device 160 has been simplified to focus on features of amemory device that are helpful in understanding the present invention.The memory device 160 includes an array 100 of non-volatile memorycells. The memory cells (not shown in FIG. 1B) are floating-gate memorycells in accordance with the embodiments of the invention. The array isarranged in rows and columns. The rows may be arranged in blocks and theerase operation may be performed on a full block in a manner similar toconventional flash memory. However, the memory cell structure and arrayorganization described herein facilitates individual erasure of selectedmemory cells independent of any block structure.

A row decoder 168 and a column decoder 170 are provided to decodeaddress signals provided on address lines A0-Ax 172. An address bufferlatch circuit 166 is provided to latch the address signals. Addresssignals are received and decoded to access the memory array 100. Aselect circuit 176 is provided to select a column of the arrayidentified with the column decoder 170. Sense amplifier and comparecircuitry 178 is used to sense data stored in the memory cells andverify the accuracy of stored data. Data input 180 and output 182 buffercircuits are included for bi-directional data communication over aplurality of data (DQ) lines 181 with the processor 161. A data latch183 is typically provided between input buffer 180 and the memory array100 for storing data values (to be written to a memory cell) receivedfrom the DQ lines 181.

Command control circuit 174 decodes signals provided on control lines173 from the processor 161. These signals are used to control theoperations on the memory array 100, including data read, data write, anderase operations. Input/output control circuit 184 is used to controlthe input buffer 180 and the output buffer 182 in response to some ofthe control signals. As stated above, the memory device 160 has beensimplified to facilitate a basic understanding of the features of thememory. A more detailed understanding of typical flash memories is knownto those skilled in the art.

Arrays of non-volatile memory cells are often arranged in rows andcolumns of memory cells coupled to word lines and bit lines,respectively. The word lines are coupled to the control gates of thefloating-gate memory cells. The bit lines are coupled to the drains ofthe floating-gate memory cells.

FIGS. 2A–2B are a planar and cross-sectional view, respectively, of anarray of field-effect transistors (FETs) in accordance with anembodiment of the invention. The cross-section of FIG. 2B is taken atline A–A′ of FIG. 2A.

The array 200 of memory cells 201 is formed on a substrate 202. For oneembodiment, the substrate 202 is a monocrystalline material, such asmonocrystalline silicon. For a further embodiment, the substrate 202 maybe doped to have a conductivity, e.g., a p-type or n-type conductivity.

Each memory cell 201 includes an FET having a gate overlying thesubstrate 202, two source/drain regions 208 and 210 in the substrate 202and a channel region defined by the area between the two source/drainregions 208 and 210. The gate of the memory cell 201 includes a controlgate provided by conductive material 220, an interlayer dielectric 218,a floating gate 216, and a tunnel dielectric 214.

A column of memory cells is a group of memory cells 201 having theirrespective first source/drain regions 208 and second source/drainregions 210 connected together. The memory cells 201 of a column ofmemory cells also share a first or shallow well 206. The first well 206has a first conductivity type. For one embodiment, the firstconductivity type is a p-type conductivity. A first well 206 of a firstcolumn of memory cells is separated from other first wells 206 byisolation trenches 204. Each first well 206 contains the firstsource/drain region 208 and the second source/drain region 210 for acolumn of memory cells. The source/drain regions 208 and 210 each have asecond conductivity type opposite the first conductivity type. Forexample, with embodiments having a p-type conductivity as the firstconductivity type, the second conductivity type is an n-typeconductivity. As the first and second source/drain regions 208 and 210are shared for a column of memory cells, no local contacts are requiredfor individual source/drain regions.

The array 200 of memory cells 201 shares a deep or second well 212underlying the first wells 206. The second well 212 has the secondconductivity type. The second well 212 has a top surface extending abovethe base of the isolation trenches 204 and a bottom surface extendingbelow the isolation trenches 204.

FIGS. 3A–3F are cross-sectional views of a portion of the memory array200 during various stages of fabrication in accordance with oneembodiment of the invention.

In FIG. 3A, isolation trenches 204 have been formed in the substrate202. The isolation trenches 204 are typically dielectric-filled trenchesusing any of a variety of shallow trench isolation (STI) techniques.These isolation trenches 204 act as insulative barriers between adjacentportions of the substrate 202.

First wells 206 are formed between the isolation trenches 204. The firstwells 206 should have a depth less than the isolation trenches 204 suchthat each first well 206 is separated from adjacent first wells 206 byan intervening isolation trench 204. However, any excessive depth of thefirst wells 206 can be overcome during the formation of the second well212 as described later herein.

As depicted in FIG. 3A, the first wells 206 are formed by doping anexposed portion of the substrate 202 between the isolation trenches 204with a first dopant species 230 of the appropriate conductivity type.For example, where the first conductivity type is a p-type, the firstdopant species 230 may have boron (B) or another p-type impurity. Or,where the first conductivity type is an n-type, the first dopant species230 may have antimony (Sb), arsenic (As), phosphorus (P) or anothern-type impurity.

As an alternative to doping the first wells 206 after formation of theisolation trenches 204, such doping may occur prior to formation of theisolation trenches 204. Doping is usually performed through ionimplantation techniques. Dopant sources for ion implantation techniquesare often fluorine-based gases. For example, in the ion implantation ofboron ions, the source gas may be boron trifluoride (BF₃). Thermalprocessing may be performed following the implantation in order todiffuse the ions and to repair surface damage caused by the ionbombardment.

In addition to ion implantation techniques, other doping methods areknown such as diffusion techniques using gaseous, liquid or solid dopantsources. Examples of dopant sources for the diffusion of boron includegaseous diborane (B₂H₆), liquid boron tribromide (BBr₃) and solid boronnitride (BN). Other dopant sources and specific techniques are wellknown in the art of semiconductor fabrication.

In FIG. 3B, first source/drain regions 208 and second source/drainregions 210 are formed. Although the first and second source/drainregions 208 and 210 are shown to be in contact with the isolationtrenches 204, they need not be.

The first and second source/drain regions 208 and 210 have a secondconductivity type different from the first conductivity type. Forexample, where the first conductivity type is a p-type conductivity, thesecond conductivity type may be an n-type conductivity. Portions of thesurface of the substrate 202 between isolation trenches 204 are coveredprior to exposing the surface of the substrate 202 to a second dopantspecies 240 of the appropriate conductivity type. For example, where thesecond conductivity type is an n-type, the second dopant species 240 hasan n-type impurity. Or, where the second conductivity type is a p-type,the second dopant species 240 has a p-type impurity. Furthermore, firstand second source/drain regions 208 and 210 generally have a higherdoping level than found in the first wells 206. For example, where thefirst wells 206 have a p-type conductivity, the first and secondsource/drain regions may have an n+doping level.

Portions of the surface of the substrate 202 are covered in FIG. 3B by apatterned mask 232, typically a photoresist material. This patternedmask 232 protects the surface of the substrate 202, and thus portions ofthe first wells 206, from exposure to the second dopant species 240. Thepatterned mask 232 thus separates the first and second source/drainregions 208 and 210 laterally to define a channel region of the futurememory cell. A sacrificial oxide layer 234 or other protective layer maybe formed between the surface of the substrate 202 and the patternedmask.

The deep or second well 212 is formed in FIG. 3C to underlie and contactthe isolation trenches 204. For one embodiment, the second well 212 isformed using a deep implant of a third dopant species 250. The thirddopant species 250 has the second conductivity type. Even where thedoping of the first wells 206 results in electrical coupling below theisolation trenches 204, formation of the second well 212 will serve toelectrically isolated adjacent first wells 206 if the doping levels aresufficient to form a layer having the second conductivity type incontact with the isolation trenches 204.

The third dopant species 250 may have the same or a different impuritythan the second dopant species. For example, the first and secondsource/drain regions 208 and 210 may be formed with an arsenic-basedimpurity while the second well 212 is formed with a phosphorus-basedimpurity. Similarly, the second dopant species 240 and the third dopantspecies 250 may have the same impurity but in a different form. Forexample, the second dopant species 240 may utilize a phosphoruspentafluoride (PF5) dopant source while the third dopant species 250utilizes a phosphorus trifluoride (PF3) dopant source. For a furtherembodiment, the doping technique may vary between forming the first andsecond source/drain regions 208 and 210 and forming the second well 212.For example, the first and second source/drain regions 208 and 210 maybe formed using a diffusion technique while the second well 212 isformed by ion implantation. The second well 212, in conjunction with theisolation trenches 204, further isolates first wells 206 from eachother.

For one embodiment, the second well 212 is formed after removal of thepatterned mask 232 and any sacrificial layer 234. For anotherembodiment, the second well 212 is formed prior to the formation of thefirst and second source/drain regions 208 and 210.

Following formation of the second well 212 and removal of the patternedmask 232 and any sacrificial layer 234, the tunnel dielectric 214 isformed overlying at least the first and second source/drain regions 208and 210 and the channel region defined between them as shown in FIG. 3D.The tunnel dielectric 214 is a dielectric material such as siliconoxides, silicon nitrides or silicon oxynitrides. For one embodiment, thetunnel dielectric 214 is grown on the surface of the substrate 202, suchas by thermal oxidation of exposed silicon areas. For anotherembodiment, the tunnel dielectric 214 is deposited on the surface of thesubstrate 202. In addition to covering the first and second source/drainregions 208 and 210 and their intervening channel regions, the tunneldielectric 214 may further extend over the isolation trenches 204.

In FIG. 3E, the floating gates 216 are formed. The floating gates 216are generally some conductive material capable of storing a charge.Conductively-doped polysilicon material is commonly used for suchfloating gates. For example, the floating gates 216 may contain ann-type polysilicon. For one embodiment, the floating gates 216 areformed by blanket deposition of a polysilicon material, conductivelydoped either during or following deposition, and patterning of thedeposited polysilicon material. The floating gates 216 should extend tobe at least overlying the channel regions defined between the first andsecond source/drain regions 208 and 210. For another embodiment, thefloating gates 216 further extend to be overlying the first and secondsource/drain regions 208 and 210. For a further embodiment, as depictedin FIG. 3E, the floating gates 216 further extend to be overlying aportion of the isolation trenches 204.

In FIG. 3F, the interlayer dielectric 218 is formed overlying thefloating gates 216 and the conductive material 220 are formed overlyingthe interlayer dielectric 218. The interlayer dielectric 218 is adielectric material such as silicon oxides, silicon nitrides or siliconoxynitrides. For one embodiment, the interlayer dielectric 218 isdeposited on the floating gates 216. While the interlayer dielectric 218need only cover the floating gates 216, it may further extend to overliethe isolation trenches 204. For another embodiment, the interlayerdielectric 218 is grown on the surface of the floating gates 216, suchas by thermal oxidation of exposed polysilicon. The conductive material220 can contain a single conductive material or conductive composite.For one example, conductively-doped polysilicon material may be used.However, it is more common to use two or more layers of conductivematerial, with at least one layer containing a metal. For one example,the conductive material 220 may contain a metal silicide layer, such astungsten silicide (WSi₂), overlying a conductively-doped polysiliconlayer. A cap layer is generally a dielectric material formed overlyingthe conductive layers of the word line stack to act as an insulator andbarrier layer. Following formation of the conductive material 220, theconductive material 220, the interlayer dielectric 218 and the floatinggate 216 are patterned, such as by etching, in a direction generallyperpendicular to the first wells 206 to define word lines for the memoryarray 200.

CONCLUSION

Floating-gate field-effect transistors or memory cells, and methods oftheir fabrication, have been described. One example of a use for suchfloating-gate memory cells is non-volatile memory arrays and devices. Acolumn of such floating-gate memory cells are associated with a wellcontaining the source/drain regions for each memory cell in the column.These wells are isolated from source/drain regions of other columns ofthe array. FN tunneling can be used to program and erase suchfloating-gate memory cells either on an individual basis or on a bulk orblock basis.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement that is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. Many adaptations ofthe invention will be apparent to those of ordinary skill in the art.Accordingly, this application is intended to cover any adaptations orvariations of the invention. It is manifestly intended that thisinvention be limited only by the following claims and equivalentsthereof.

1. An electronic system, comprising: a processor; and a non-volatilememory device coupled to the processor, wherein the non-volatile memorydevice comprises: an array of non-volatile floating-gate memory cellsarranged in rows and columns; and control circuitry for controllingaccess to the array of memory cells; wherein each column of memory cellsshares a source and a drain, the source and drain for a column of memorycells being contained in a first well associated with that column ofmemory cells; wherein the first well associated with each column ofmemory cells is isolated from other first wells of other columns ofmemory cells; wherein each first well is overlying a second well in amany-to-one relationship; wherein each first well has a firstconductivity type; wherein the second well has a second conductivitytype different than the first conductivity type.
 2. The electronicsystem of claim 1, wherein the first conductivity type is a p-typeconductivity and the second conductivity type is an n-type conductivity.3. The electronic system of claim 1, wherein the first wells areisolated from each other by isolation trenches interposed betweenadjacent first wells and by the second well.
 4. The electronic system ofclaim 3, wherein the second well is underlying and in contact with theisolation trenches.